Because the interaction between myosin and ATP has a central role in muscle contraction, it is necessary to elucidate this interaction before an understanding of the molecular basis of contraction can be achieved. Our objectives are to determine the coordination scheme by which the substrate and metal bind to myosin, to determine the role of Mg 2 ion and that of the conformational change associated with hydrolysis of ATP in retarding the dissociation of products, to determine what chemical reactions may influence the motion of the crossbridges of synthetic myosin filaments, and to determine how these properties may differ in myosins from cardiac muscle, fast skeletal muscle and slow skeletal muscle. The coordination scheme will be determined by measuring proton relaxation rates of water protons in the presence of a paramagnetic metal, Mn 2 ion, and from the esr spectrum of Mn 2 ion. We will determine the effects of activators of the ATPase activity on the binding of Mn 2 ion and on the conformational change associated with ATP hydrolysis measured by esr or fluorescence of spin labeled and native myosin, respectively. Saturation transfer esr spectroscopy will be used to measure the rotational correlation times of a spin label bound to the heads (crossbridges) of synthetic myosin filaments.